A Knowledge Summary in Veterinary Evidence, published in December, states that there is “very weak evidence” that adding transdermal nitroglycerin to other therapies “speeds the resolution of clinical signs” in the condition.

Usage of nitroglycerin varies between practices but this conclusion means those that apply it regularly should be cautious and use other treatments where applicable, especially considering the lack of research into the drug’s potential side effects.

Dr Søren Boysen, from the Department of Veterinary Clinical and Diagnostic Sciences at the University of Calgary, said: “Based on the results of the Knowledge Summary, it is not wrong to use transdermal nitroglycerin in the management of left-sided congestive heart failure, but it should never be the sole agent and should only be used as a supplemental therapy to other proven therapies.”

...those that apply it regularly should be cautious and use other treatments where applicable

Not a long-term treatment option

The evidence, which amounts to four studies from 1995 to 2014, demonstrates that nitroglycerin does indeed have a decreasing effect on blood pressure (and associated indicators of cardiac health such as systemic vascular resistance) as expected due to the drug’s vasodilative action.

However, much of the research was carried out on healthy animals, under anaesthetic or induced circumstances, which may mean the clinical signs measured had a different pathophysiology to those of naturally occurring heart disease and, as such, response to the drug could differ in practice cases.

A further obstacle to consider is the tolerance effect exhibited by treatment with nitrates such as nitroglycerin. In humans, long-term use of this class of drug can lead to a reduction in its effectiveness, only reset when it is periodically removed.

Therefore, and taking into account that there are more efficient treatment options for removing fluid from the lungs of dogs with congestive heart failure (like diuretics), nitroglycerin can’t currently be recommended for the management of this condition.

“Although transdermal nitroglycerin has an effect on blood pressure in the research setting, this may not equate to decreased pulmonary oedema or resolution of clinical signs in dogs with left-sided congestive heart failure,” said Dr Boysen.

“If the objective is longer-term control, then oral medications would be a more effective approach.”

Emergency use

Nitroglycerin is fast acting and, according to the evidence, causes significant vasodilation in somewhere between under ten minutes and one hour.

To exemplify its effect, application of nitroglycerin tape at 2.5mg/kg in one study caused a 10 to 15 percent decrease in systolic blood pressure, sustained until removal of the tape. In theory therefore, it is possible that transdermal nitroglycerin could provide a benefit in patients with left-sided congestive heart failure by rapidly lowering hydrostatic pressure (indirectly reflected by blood pressure) through vasodilation, thereby decreasing fluid shifts from the vascular to the pulmonary interstitial/alveolar spaces.

As such, dogs with an urgent need of lower hydrostatic pressures can be treated with transdermal nitroglycerin. However, agents such as intravenous nitroprusside are more effective.

Understudied side effects

Nitroglycerin has a number of known side effects in humans, ranging from headache, dizziness and nausea to serious complications like severe hypotension and a low heart rate.

Although no side effects were noted in the evidence, studies investigating adverse reactions in dogs are lacking. Practitioners should be aware of this when using the drug.

Jennifer Parker, Bsc, MSc, PgDip, is Editor of Veterinary Practice magazine. She studied zoology at the University of Reading, palaeoanthropology at University College London and completed a postgraduate diploma in endangered species recovery with Durrell Wildlife Trust and the University of Kent.